1. Field of the Invention
The present invention is directed to a cathode system for an x-ray tube of the type having an electron emitter and a further electrode arranged between electron emitter and anode.
2. Description of the Prior Art
Cathode systems currently utilized in x-ray tubes have reached the limits of their development with respect to service life and focus shape. A longer service life and what is referred to as a "Gaussian" focus occupation, however, will be desired in future. Gaussian focus occupation means that the intensity of the electrons incident onto the x-ray tube anode, and thus the intensity of the X-radiation emanating from the focus, are maximum in the center of the focus and decrease toward the edge, analogously to a Gaussian bell curve.
A cathode system of the initially cited type is generally known from the work of Pierce in the form of an electrode system which at least partially meets these demands. The further electrode of the known electrode system is an auxiliary anode that is shaped such that the influences of the space charges of the electron beam are compensated. A disadvantage of this system is that the voltages at the anode, the auxiliary anode and the electron current (tube current) are linked to one another. The tube current, thus, is no longer freely selectable; this means that, for example, a specific tube current is associated with a specific tube voltage. Theoretically, it would be conceivable to alleviate this disadvantage by introducing a further, cathode-proximate control grid. This however, could only be technologically realized in a very complicated way.
X-ray tubes having a cathode system of the type initially cited are disclosed in German OS 34 26 623 and German OS 35 14 700.
Further, German OS 32 28 816 discloses a method for x-ray computed tomography wherein keyed x-ray pulses that are generated by keying the electron current in the x-ray tube with a Wehnelt cylinder, are employed for achieving an improved signal-to-noise ratio without increasing the radiation dose.